This invention relates to cured fluorine-containing materials having a refractive index of up to 1.335 at 25xc2x0 C. and best suited as optical materials such as antireflection films as well as rubber materials, tent film materials, sealants, coating materials, and parting agents where solvent resistance is required.
Heretofore, curable fluorine-containing compositions primarily comprising a polymer of fluorine-containing organic compound and a crosslinking agent have been used in a variety of applications.
With the rapid development of the information society, large size displays of the liquid crystal, CRT, plasma and other systems are on widespread use. Those displays, especially of the portable type, used outdoor or in an illuminated space are required to improve their recognition capability. One common means for improving the recognition capability is to provide the substrate of a display device with an antireflection film of low refractive index materials, typically fluorine compounds. Such antireflection films are formed by vacuum evaporating inorganic materials or by depositing alternating films of high and low refractive index materials, which techniques lack productivity.
Known low refractive index materials which can be coated have a refractive index of 1.34 at the lowest. There is a need for a material having a lower refractive index.
An object of the invention is to provide a cured fluorine-containing material having a sufficiently low refractive index to be used as an antireflection film or the like.
It has been found that a curable composition comprising (A) a linear fluoropolyether compound having at least two alkenyl groups per molecule and a perfluoroalkyl ether structure in the backbone, (B) a fluorine-containing organohydrogensiloxane, and (C) a platinum group catalyst can be coated in solution form and converted, by holding at room temperature or heating, into a cured thin film comprising a perfluoropolyether backbone and having a refractive index of up to 1.335 at 25xc2x0 C., especially when the fluorine content in the cured film is at least 61.0% by weight.
Briefly stated, the invention provides a cured fluorine-containing material having a refractive index of up to 1.335 at 25xc2x0 C. comprising as the backbone a perfluoropolyether of the following general formula (1):
xe2x80x94(Rfxe2x80x94O)qxe2x80x94xe2x80x83xe2x80x83(1) 
wherein Rf is a perfluoroalkylene group of 1 to 6 carbon atoms and q is a number of 1 to 500.
The cured fluorine-containing material of the invention is obtained by curing a curable composition comprising (A) a linear fluoropolyether compound having at least two alkenyl groups per molecule and a perfluoroalkyl ether structure of the formula (1) in the backbone, (B) a fluorine-containing organohydrogensiloxane, and (C) a platinum group catalyst.
The linear fluoropolyether compound (A) should have at least two alkenyl groups per molecule and a perfluoroalkyl ether structure in the backbone. It is used as a base polymer in the composition.
The alkenyl groups in the linear fluoropolyether compound are, for example, groups having a CH2xe2x95x90CHxe2x80x94 structure at the terminus such as vinyl, allyl, propenyl, isopropenyl, butenyl and hexenyl, and preferably vinyl and allyl. The alkenyl groups may be attached to the backbone of linear fluoropolyether compound at opposite ends directly or through divalent linking groups such as xe2x80x94CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94 or xe2x80x94Yxe2x80x94 NRxe2x80x94COxe2x80x94. Herein Y is xe2x80x94CH2xe2x80x94 or: 
(wherein the free valence bond may be at an o-, m- or p-position), and R is hydrogen, methyl, phenyl or allyl.
The perfluoroalkyl ether structure in the linear fluoropolyether compound is of the general formula (1) as mentioned above.
xe2x80x94(Rfxe2x80x94O)qxe2x80x94xe2x80x83xe2x80x83(1) 
Rf is a straight or branched perfluoroalkylene group of 1 to 6 carbon atoms, preferably 1 to 3 carbon atoms, and q is an integer of 1 to 500, preferably 2 to 400, and more 3 preferably 10 to 200.
Examples of the recurring units represented by xe2x80x94(Rfxe2x80x94O)xe2x80x94 include xe2x80x94CF2xe2x80x94, xe2x80x94CF2CF2Oxe2x80x94, xe2x80x94CF2CF2CF2Oxe2x80x94, xe2x80x94CF(CF3)CF2Oxe2x80x94, xe2x80x94CF2CF2CF2CF2Oxe2x80x94, xe2x80x94CF2CF2CF2CF2CF2CF2Oxe2x80x94, and xe2x80x94C(CF3)2Oxe2x80x94, Preferred among these are xe2x80x94CF2xe2x80x94, xe2x80x94CF2CF2Oxe2x80x94, xe2x80x94CF2CF2CF2Oxe2x80x94, and xe2x80x94CF(CF3)CF2Oxe2x80x94. Especially preferred are perfluoropolyethers comprising recurring units of hexafluoropropenoxide. The perfluoroalkyl ether structure may consist of one or more types of recurring units represented by xe2x80x94(Rfxe2x80x94O)xe2x80x94.
Examples of the linear fluoropolyether compound (A) are linear fluoropolyether compounds of the following general formula (2): 
wherein X is independently xe2x80x94CH2xe2x80x94, xe2x80x94CH2Oxe2x80x94, or xe2x80x94Yxe2x80x94NRxe2x80x94COxe2x80x94, Y is xe2x80x94CH2xe2x80x94 or: 
(wherein the free valence bond may be at an o-, m- or p-position), R is hydrogen, methyl, phenyl or allyl, p is independently 0 or 1, k is an integer of 2 to 6, m and n each are an integer of 0 to 200, preferably 5 to 150, and having a weight average molecular weight of about 400 to 100,000, preferably about 2,000 to 50,000.
Illustrative, non-limiting examples of the linear fluoropolyether compound (A) are shown below: 
In the above formulas, m and n are as defined in formula (1), Me is methyl and Ph is phenyl.
These linear fluoropolyether compounds may be used alone or in admixture of two or more.
The linear fluoropolyether compound (A) used herein may range from a low viscosity polymer having a viscosity of several ten centistokes at 25xc2x0 C. to a solid gum-like polymer. From the ease of handling standpoint, polymers having a viscosity of about 1,000 to 100,000 centistokes at 25xc2x0 C. are advantageously used for coating purposes. Polymers having a too low viscosity may result in cured films having reduced film strength and/or adhesion, failing to provide a good profile of physical properties.
The fluorine-containing organohydrogensiloxane (B) serves as a crosslinking agent or chain extender for the linear fluoropolyether compound (A). The fluorine-containing organohydrogensiloxane is not critical as long as it has at least one monovalent perfluoroalkyl group, monovalent perfluorooxyalkyl group, divalent perfluoroalkylene or divalent perfluorooxyalkylene group and at least two, preferably at least three hydrosilyl groups, i.e., Sixe2x80x94H groups in a molecule. The perfluoroalkyl, perfluorooxyalkyl, perfluoroalkylene and perfluorooxyalkylene groups are exemplified by the groups of the following general formulae.
monovalent perfluoroalkyl groups:
CpF2p+1xe2x80x94
Letter p is an integer of 1 to 20, preferably 2 to 10. divalent perfluoroalkylene groups:
xe2x80x94CpF2pxe2x80x94
Letter p is an integer of 1 to 20, preferably 2 to 10. monovalent perfluorooxyalkyl groups: 
Letter q is an integer of 1 to 5.
divalent perfluorooxyalkylene groups: 
The sum of r+s is an integer of 2 to 100 on average.
The divalent linking group which links the perfluoroalkyl, perfluorooxyalkyl, perfluoroalkylene or perfluorooxyalkylene group to the silicon atom is an alkylene group, arylene group or a mixture thereof, which may further have an ether bond oxygen atom, amide bond or carbonyl bond. Such divalent linking groups of 2 to 12 carbon atoms are preferred. Illustrative examples thereof include
xe2x80x94CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2xe2x80x94, xe2x80x94CH2CH2CH2OCH2xe2x80x94,
xe2x80x94CH2CH2CH2xe2x80x94NHxe2x80x94COxe2x80x94, xe2x80x94CH2CH2CH2xe2x80x94N(Ph)xe2x80x94COxe2x80x94,
xe2x80x94CH2CH2CH2xe2x80x94N(CH3)xe2x80x94COxe2x80x94, and xe2x80x94CH2CH2CH2xe2x80x94Oxe2x80x94COxe2x80x94
wherein Ph is phenyl.
In addition to the monovalent organic group containing a monovalent or divalent fluorinated substituent, that is, a perfluoroalkyl, perfluorooxyalkyl, perfluoroalkylene or perfluorooxyalkylene group, the fluorine-containing organohydrogensiloxane (B) may have a monovalent substituent attached to a silicon atom. Exemplary monovalent substituents are substituted or unsubstituted hydrocarbon groups of 1 to 20 carbon atoms including alkyl groups such as methyl, ethyl, propyl, butyl, isobutyl, tert-butyl, pentyl, hexyl, octyl and decyl; cycloalkyl groups such as cyclopentyl, cyclohexyl and cycloheptyl; alkenyl groups such as vinyl, allyl, propenyl, isopropenyl, butenyl and hexenyl; aryl groups such as phenyl, tolyl, xylyl and naphthyl; aralkyl groups such as benzyl, phenylethyl and phenylpropyl; and substituted ones of these groups in which some of the hydrogen atoms are replaced by chlorine atoms, cyano groups or the like, such as chloromethyl, chloropropyl, and cyanoethyl.
The fluorine-containing organohydrogensiloxane may be cyclic, chainlike or three-dimensional network. The number of silicon atoms in the molecule of the fluorine-containing organohydrogensiloxane is desirably about 2 to about 200, and especially about 3 to about 150, though not limited thereto.
Illustrative examples of the fluorine-containing organohydrogensiloxane are given below. They may be used alone or in admixture of two or more. In the formulae, Me is methyl and Ph is phenyl. 
Usually, component (B) having hydrosilyl groups is blended in such an amount that 0.5 to 5 mol, and more preferably 1 to 2 mol of the hydrosilyl (or SiH) groups may be available per mol of aliphatic unsaturated groups (e.g., vinyl, allyl or cycloalkenyl) in the entire composition. Less than 0.5 mol of hydrosilyl groups may achieve an insufficient degree of crosslinking. More than 5 mol of hydrosilyl groups may allow chain lengthening to become preferential, inviting short curing, foaming, and losses of heat resistance and the like. An appropriate amount of component (B) blended is usually 0.1 to 50 parts, preferably 0.1 to 20 parts by weight per 100 parts by weight of component (A).
The platinum group catalyst (C) is a catalyst for promoting the addition reaction or hydrosilylation of the fluoropolyether compound with the fluorine-containing organohydrogensiloxane. Exemplary catalysts are chloroplatinic acid, alcohol solutions of chloroplatinic acid, aldehyde solutions of chloroplatinic acid, and complex salts of chloroplatinic acid with olefin compounds.
The platinum group catalyst is added in an amount necessary to promote the addition reaction, usually in such an amount as to provide 10 to 500 ppm of platinum group metal based on component (A).
Various well-known additives may be added to the inventive composition for improving its practical usage insofar as they do not interfere with curing. For example, there may be added addition reaction regulators for controlling the activity of the platinum group catalyst (C), such as organic nitrogen compounds, organic phosphorus compounds and acetylene compounds. It should be avoided to add those additives which cause the cured part to have a refractive index in excess of 1.335.
It is not critical how to prepare the cured fluorine-containing material according to the invention. Typically it is prepared by compounding the above-described components and heat curing the compound.
On practical usage, the cured fluorine-containing material of the invention is obtained by applying a composition comprising components (A) to (C) onto a substrate, and curing the composition to the substrate surface. If desired, the composition is dissolved in a fluorochemical solvent such as meta-xylene hexafluoride or florinate before it is applied. The substrate may be of glass or various plastics. Any well-known technique may be employed to apply the composition to the substrate. The curing conditions for the composition may be selected as appropriate. Room temperature curing is possible depending on the type of components (A), (B) and (C) although heat curing at 50 to 150xc2x0 C. for several ten seconds to several ten minutes is preferred.
The cured fluorine-containing material of the invention should have a refractive index at 25xc2x0 C. of up to 1.335, preferably 1.300 to 1.335, and more preferably 1.310 to 1.330. Since this belongs to the lowest refractive index class of organic materials, the inventive material can be advantageously used as an optical material such as antireflection film material. The cured material of the invention has improved solvent resistance and chemical resistance due to its high fluorine content, and improved parting properties and water repellency due to its low surface energy. Since the starting composition can be diluted with a solvent, the cured fluorine-containing material is obtainable at low cost. For these reasons, the cured fluorine-containing material of the invention is also advantageously used in the application where solvent resistance is required, typically as rubber materials, tent film materials, sealants, coating materials, and parting agents.
It is preferred for achieving a lower refractive index that the cured fluorine-containing material of the invention have a fluorine content of at least 61.0%, more preferably 61 to 75%, and most preferably 62 to 70% by weight.
There has been described a cured fluorine-containing material having a lower refractive index and useful as antireflection film or the like.